The present invention relates to a method of manufacturing a sandwich panel by in situ foaming.
Such a method is known in the art, for example from EP-A1-0636463. A sandwich panel generally consists of a foamed core layer, that is covered by two cover layers. The cover layer(s) may comprise (fiber-reinforced) thermoplastics, metal and the like, or combinations thereof. These cover layers are also called facings in the respective art. The in situ foaming process consists of several steps. The first step is an assembling step wherein a core web comprising at least one sheet of a thermoplastic material containing an amount of a suitable physical blowing agent is provided. Subsequently this core web is positioned between e.g. two (fiber-reinforced) cover layers, usually of a similar thermoplastic plastic material. Then the assembly of core web and cover layers is arranged between two press plates in a press. In this state, a foaming step is carried out, wherein heat and pressure are supplied to the press to cause foaming of the core web and simultaneously effecting a bond thereof to the cover layers. During this foaming step, when the temperature has become high enough, the press is slowly opened thereby increasing the distance between the two press plates. This allows for the liquid solvent or blowing agent to expand thereby foaming the core web material. This expansion is performed under generally controlled conditions. In this way, the core web is foamed and the bond between the core web and the facing(s) is generated in one production step without the need for a separate or additional adhesive to be used. When a predetermined foamed core layer thickness is obtained, the foamed assembly is allowed to cool during a cooling step. The foamed assembly thus obtained comprises the foamed core layer covered by the two cover layers. In the in situ foaming process it is essential that physical blowing agent is prevented from being diffused through the facing during the foaming phase as well as during a main part of the cooling phase. Even very tiny amounts passed through a facing during these phases may result in local surface irregularities, foam collapse and/or insufficient adhesion.
If a physical blowing agent comprising a solvent, swelling agent or combination thereof has been used, in general the manufacturing method is completed by a drying step wherein the thus obtained sandwich panel is dried at elevated temperature for a time period sufficient to reduce the level of physical blowing agent, for example by heating the sandwich in an oven. Reduction of the level of physical blowing agent is necessary to achieve a reduction of inflammability that is required for most intended applications, such as lightweight construction panels for aircrafts and the like.
In practice, the known method of manufacturing a sandwich panel comprising at least one fiber-reinforced cover layer by in situ foaming has presented excellent results for relatively small sandwich panels, for example in the order of 25×25 cm at most. Even larger panels have been manufactured using a solvent like methylene chloride (MC). However, it has been found that if sandwich panels are manufactured having larger dimensions, for example 50×32 cm, during scale-up using the same in situ foaming method but using a swelling agent (or a mixture of a swelling agent and a solvent or a solvent having a very low solubility for the respective thermoplastic), the quality of the sandwich panels leave something to be desired. In particular, the facings of the final product showed certain surface irregularities in the form of bulges and/or pits. These surface irregularities are present over the whole surface of the cover layer(s), but frequently at the peripheral edge area. It is expected that even larger sandwich panels made by in situ foaming e.g. having standard dimensions used in industry, would exhibit the same surface irregularities, if a swelling agent, a mixture of a swelling agent and a solvent or a solvent having a very low solubility is used.